In modern, water-using household appliances, in particular, in program-controlled dishwashers, turbidity sensors are used to optimize the wash-program sequence. Ideally, during a measuring interval, a turbidity sensor which, in particular, is configured with optical means sends signals to an evaluation device provided in the appliance, these signals including signals representative of the degree of discoloration or of the transparency of the wash water as well as signals representative of the number of dirt particles detected in the wash water during the measuring interval. Based on the degree of discoloration and the number of dirt particles, the evaluation device then calculates the degree of turbidity which, in turn, influences the program sequence of the appliance. However, in practice, it turned out that the measuring zone of the turbidity sensor is also traversed by air bubbles and foam. However, the turbidity sensor cannot differentiate between air bubbles and dirt particles. Therefore, the turbidity sensor sends faulty signals for the number of dirt particles to the evaluation device, making it more difficult to calculate an exact degree of turbidity. A comparable problem occurs in washing machines when foam bubbles increasingly accumulate in the washing liquid.
In the earlier German Patent DE 100 59 112 C1, the applicant proposed to solve these problems by moving the circulating wash water at a reduced flow velocity within a measuring zone. In this connection, the measuring zone, together with the turbidity sensor, is disposed in a horizontally extending wash water conduit section which continuously widens upward with respect to the horizontal plane and forms a stilling section for the fluid flow. In the space of reduced flow velocity, it is possible for air bubbles to accumulate in a plane above the measuring zone and to be removed with the flow.
However, this approach was found to have the disadvantage that when the circulating pump moving the wash water operates at full flow, the dwell time of the wash water in the stilling section is very short so that there are still some air bubbles and foam passing through the measuring zone.
In German Patent DE 101 46 641 C1, the applicant proposed to solve the problem by a device having a main flow conduit and a parallel bypass conduit which is connected to the main flow conduit and features an enlarged region of reduced flow velocity in which is located the measuring zone for the turbidity sensor.
This was an improvement over the prior art described earlier, but tests have shown that, occasionally, there are still air bubbles traversing the measuring zone during the washing operation.